In the conventional technique, a prism sheet is used so that the traveling direction of the light emitted from a surface illuminating device is condensed in the normal direction of the light emitting surface of the surface illuminating device, whereby the luminance in the aforementioned normal direction is increased. The prism sheet of this type is structured in such a way that triangular prisms and square pyramids are arranged without any gap on one side of the transparent flat plate, for example. The slope angle is adjusted to an appropriate level to obtain the intended optical path change characteristic, which is disclosed in Tokkai 2005-55481. The same Document discloses that this prism sheet does not lose substantial functions when the square pyramid is replaced by a truncated square pyramid.
However, the present inventors conducted various forms of simulation on the shape of the prism sheet and have found out the following: It is difficult that the desired optical path change characteristic is always obtained merely by adjusting the slope angle of the triangular prism or square pyramid, and that the light from the surface light emitting device is easily picked up efficiently in the normal direction of the light emitting surface, viz., in the front direction.